In the past 540 million years, Earth has endured five mass extinctions that led to the widespread extermination of marine species around the world. Each involved processes that upended the normal cycling of carbon through the atmosphere and oceans, unfolding over thousands to millions of years. 

The question for many scientists is whether the carbon cycle is now experiencing a significant jolt that could tip the planet toward a sixth mass extinction. In the modern era, carbon dioxide emissions have risen steadily since the 19th century, but deciphering whether the recent spike could lead to mass extinction has been challenging. That’s mainly because it’s difficult to relate ancient carbon anomalies, occurring over thousands to millions of years, to today’s disruptions, which have taken place over a little more than a century.

Now Daniel Rothman, professor of geophysics in the Department of Earth, Atmospheric, and Planetary Sciences and co-director of MIT’s Lorenz Center, has analyzed significant changes in the carbon cycle over the last 540 million years, encompassing the five mass extinction events. He has identified “thresholds of catastrophe” in the carbon cycle that, if exceeded, would lead to an unstable environment and, ultimately, another extinction.

In a paper published in Science Advances, he proposes that mass extinction occurs under two conditions. For changes in the carbon cycle that occur over long time scales, extinctions will follow if those changes occur faster than global ecosystems can adapt. For perturbations that take place over shorter time scales, the pace of change will not matter; instead, the size or magnitude of the change will determine the likelihood of an extinction event. 

Taking this reasoning forward in time, Rothman predicts that given the recent rise in carbon dioxide emissions over a relatively short time scale, the key question is whether a critical amount of carbon is added to the oceans. That amount, he calculates, is about 310 gigatons, which he estimates is roughly equivalent to the amount of carbon that human activities will have added to the world’s oceans by the year 2100.

Does this mean mass extinction will soon follow? Rothman says it would take some time—about 10,000 years—for such ecological disasters to play out. However, he says that by 2100 the world may have tipped into “unknown territory.”

“This is not saying that disaster occurs the next day,” Rothman says. “It’s saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction.”